(1) Field of the Invention
The invention relates to the general field of silicon integrated circuits, more particularly to Dynamic Random Access Memories and capacitors used therein.
(2) Description of the Prior Art
Dynamic Random Access Memories (DRAMs), as used in silicon integrated circuits, represent semiconductor devices that offer information storage at very high densities. These high densities are a consequence of the fact that the basic cell of a DRAM (wherein a single bit of information is stored) comprises a single Field Effect Transistor (FET) and a capacitor. The minimum capacitance required for the basic cell to operate efficiently is about 20 femtofarads. This implies that the capacitor, rather than the FET, will be the limiting factor in determining how small the DRAM's basic cell can be made.
The value of a capacitor can be increased in any of three ways: the dielectric layer may be made thinner, its dielectric constant may be increased, and the area of the electrodes may be increased. The first two ways are limited by the availability of suitable materials with suitable properties such as breakdown strength, high frequency characteristics, etc. so that most of the work to reduce the physical size of the capacitor in a DRAM cell has concentrated on increasing the effective area of the electrodes without increasing the amount of space within the integrated circuit that is taken up by the capacitor.
One of the geometries that has been successfully used to increase the effective area of the capacitor electrodes is to shape the capacitor in the form of a rectangular prism, at least one of whose surfaces is dimpled in some way such as rectangular or cylindrical depressions. The comb-type capacitor is an example of this and is the geometry on which the present invention is based.
A number of processes for manufacturing a comb-type capacitor have been described in the prior art but none of these is completely satisfactory in that they are either unduly complicated or incorporate procedures that have the potential to lead to problems at a later stage. For example, Kwon et al. (U.S. Pat. No. 5,364,809 Nov. 15, 1994) describes a process that depends on the formation of oxide spacers that are supported by photoresist.
Other examples of patents that relate to comb-type capacitors include that of Dennison (U.S. Pat. No. 5,292,677 Mar. 8 1994) which relates to the DRAM cell as a whole, rather than to the capacitor, Kimura et al. (U.S. Pat. No. 5,374,576 Dec. 20, 1994) which teaches a modification of the capacitor geometry rather than an improved process for manufacturing it, and Lu et al. (U.S. Pat. No. 5,231,052 Jul. 27, 1993) which is concerned with the problem of making good electrical contact to a DRAM capacitor. Ajika et al. (U.S. Pat. 5,364,811 Nov. 15, 1994) describes the formation of a projecting rim on the surface of the capacitor (which is also a feature of the present invention). However, the rim described by Ajika is limited to one side of the capacitor whereas the rim that is described in the present invention runs around on four sides.